this "fuel gauge to empty" and warning lamp on is a common complaint after fill-ups is a common and new problem on later BMW's.....the claim is that sulfurs in the fuel causes a build-up on the sensor contacts and is agrevated during the filling process. having been a BMW tech. for so long though, I 'know' it's a design/material/vender problem and BMW's fix is sensor replacement but as with the early M-60 engine failures, we were replace the culpret with the identical component [short block] until an engineering solution had time to reach us.